Polyamides have been widely used, in particular, for industrial applications on account of their properties. However, the rigidity of polyamides is too high for numerous applications, so they are frequently rendered more flexible by the incorporation of plasticizers. Plasticized polyamides are typically found in tubes of the type used in the automobile industry as gasoline lines or in refrigeration units to carry coolants, such as freon. They are also used for the production of shoe soles for sports shoes, as well as skiing and climbing boots.
The plasticizers commonly used have a marked tendency to migrate, and this is a disadvantage because it can lead to the formation of coatings on the plastic articles formed or to a reduction in the flexibility thereof. This migration occurs to a particularly marked extent if polyamide 6 is rendered flexible with caprolactam as plasticizer. Further details about the behaviour of plasticizers in polyamides are given in Vieweg/Muller "Kunststoff-Handbuch" (Volume VI), Polyamide, Carl-Hanser-Verlag, Munich 1966.
Polycarbodiimides are known as anti-ageing agents for ester group-containing plastic materials (see BE-PS 610,969 and 612 040) and they are also used as oxidation and hydrolysis stabilizers (JP-PA 50-44 and US-PS 3,835,098). In connection with polyamides, DE-PS 1 950 590 describes the use of carbodiimides as co-catalysts during catalytically induced polymerization which takes place within a few minutes. According to DE-OS 2 801 701 and US-PS 4,128,599, the stability of the melt and the viscosity are increased by reaction of the terminal polymer groups with aromatic carbodiimides which either have no substituents or have a single methyl group.